Thermoplastic vulcanizates include blends of dynamically cured rubber and thermoplastic polymers. The rubber may be dispersed within the thermoplastic resin phase as finely-divided rubber particles. These compositions have advantageously demonstrated many of the properties of thermoset elastomers, yet they are processable as thermoplastics. Because thermoplastic vulcanizates are advantageously processable as thermoplastics, elastomeric articles are often extruded from thermoplastic vulcanizates using thermoplastic extruding techniques.
Thermoplastic vulcanizates may advantageously be prepared by dynamically vulcanizing a rubber with a phenolic resin while the rubber is being mixed with a thermoplastic resin. U.S. Pat. No. 4,311,628 teaches thermoplastic vulcanizates including fully-cured rubber can be produced by using phenolic resin cure systems that include a phenolic curing resin and a cure activator such as stannous chloride. According to the present invention, from 5 to 20 parts by weight phenolic resin and 0.1 to 10 parts by weight cure activator, per 100 parts by weight rubber, may be used. Likewise, U.S. Pat. No. 5,952,425 teaches the use of halogen donors in an amount from about 0.01 to 10 parts by weight per 100 parts by weight rubber. The stannous chloride may be employed in an excess of one part by weight per 100 parts by weight rubber in order to achieve technologically useful products that include fully-cured rubber.
Disadvantageously, it is believed that the stannous chloride leads to several drawbacks. For example, the stannous chloride contributes to the hygroscopicity of thermoplastic vulcanizates. Attempts have been made to reduce the propensity of these thermoplastic vulcanizates to attract and absorb atmospheric moisture. Unfortunately, these attempts have not met with a great degree of success. Where a cure activator such as stannous chloride is completely removed from the system, the ability to fully cure the rubber is precluded as suggested in U.S. Pat. No. 6,143,828. As a result, thermoplastic vulcanizates that exhibit the advantageous properties associated with fully-cured rubber cannot be attained. U.S. Pat. No. 5,952,425 also suggests that cure kinetics are not technologically useful in the absence of stannous chloride. Other attempts to alleviate the problem have included the use of alternate activator systems. For example, U.S. Pat. No. 6,437,030 teaches the use of reduced-halide systems based on metal carboxylates. Unfortunately, these systems tend to have slower cure kinetics and often result in lower cure states. Also, these systems often require preventative measures to compensate for drastic reductions in heat stability that have been observed. As for shelf life, current attempts to minimize the problem have focused on the packaging of the thermoplastic vulcanizates, where special foil liners are often employed. This is not only costly, but also it is not a solution to the problem.
In addition to problems associated with hygroscopicity, thermoplastic vulcanizates prepared with phenolic resin cure systems with a stannous chloride cure activator have exhibited a propensity to plate out, which refers to residue buildup within extrusion dies that can eventually affect engineering tolerances. Therefore, conventional extrusion practices often require routine cleaning of the dies. The prior art has not offered any explanation as to why this plate out occurs and, moreover, has not offered any solution to the problem.
U.S. Pat. No. 7,439,304 teaches thermoplastic vulcanizates having at least 0.2 parts by weight and less than 0.9 parts by weight stannous chloride stannous chloride, per 100 parts by weight rubber. Below 0.2 parts by weight, the thermoplastic vulcanizates do not have adequate heat resistance and above these amounts the thermoplastic vulcanizates show appreciable plate-out and have a deleterious impact on the hygroscopicity of the thermoplastic vulcanizates. Within this range, an advantageous cure was discovered while taking advantage of the improved hygroscopicity resulting from the reduced use of stannous chloride. Also, the extruded and molded parts prepared from these thermoplastic vulcanizates showed less porosity after extended storage even without special packaging.
Still other problems include those associated with manufacturing the thermoplastic vulcanizates with the stannous chloride. The stannous chloride is a solid material, often in the form of a powder, that is difficult to handle and not easily metered into the reactors where dynamic vulcanization takes place. Also, the stannous chloride can be difficult to disperse throughout the blend of the rubber and thermoplastic resin. It is furthermore believed that stannous chloride participates in certain side reactions that lead to poor surface appearance of extruded thermoplastic vulcanizates.
Because phenolic resin cure systems provide significant advantages over other cure systems that may be employed in the production of thermoplastic vulcanizates, there is a continued need to alleviate some of the problems associated with the use of these cure systems in the production of thermoplastic vulcanizates.